1. Technical Field
Molecular glass photoresists are disclosed for use in semiconductor lithography processes. The disclosed molecular glass photoresists include a tetrahedral structure comprising a silicon core atom with sp3 orbitals of the silicon core atom. The tetrahedral structure of the disclosed glass resists provides a diamond-like 3-D architecture, a glass transition temperature (Tg) well above room temperature and excellent glass forming properties without the need to incorporate Tg-suppressing aliphatic chains to prevent crystallinity.
2. Description of the Related Art
To meet the requirements for faster performance, integrated circuit devices continue to get smaller and smaller. The manufacture of integrated circuit devices with smaller features introduces new challenges in many of the fabrication processes conventionally used in semiconductor fabrication. One fabrication process that is particularly impacted is photolithography.
In semiconductor photolithography, photosensitive films in the form of photoresists are used for transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist-coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate.
A photoresist can be either positive-acting or negative-acting. With a negative-acting photoresist, the exposed coating layer portions polymerize or crosslink in a reaction between a photoactive compound and polymerizable reagents of the photoresist composition. Consequently, the exposed portions of the negative photoresist are rendered less soluble in a developer solution than unexposed portions. In contrast, with a positive-acting photoresist, the exposed portions are rendered more soluble in a developer solution while areas not exposed remain less soluble in the developer.
Chemically-amplified-type resists are used for the formation of sub-micron images and other high performance, smaller sized applications. Chemically-amplified photoresists may be negative-acting or positive-acting and generally include many crosslinking events (in the case of a negative-acting resist) or de-protection reactions (in the case of a positive-acting resist) per unit of photogenerated acid (PGA). In the case of positive chemically-amplified resists, certain cationic photoinitiators have been used to induce cleavage of certain “blocking” groups from a photoresist binder, or cleavage of certain groups that comprise a photoresist binder backbone. Upon cleavage of the blocking group through exposure of a chemically-amplified photoresist layer, a polar functional group is formed, e.g., carboxyl or imide, which results in different solubility characteristics in exposed and unexposed areas of the photoresist layer.
While suitable for many applications, currently available photoresists have significant shortcomings, particularly in high performance applications, such as formation of sub-half micron (<0.5 μm) and sub-quarter micron (<0.25 μm) patterns. Currently available photoresists are typically designed for imaging at relatively higher wavelengths, such as G-line (436 nm), I-line (365 nm) and KrF laser (248 nm) are generally unsuitable for imaging at short wavelengths such as sub-200 nm. Even shorter wavelength resists, such as those effective at 248 nm exposures, also are generally unsuitable for sub-200 nm exposures, such as 193 nm. For example, current photoresists can be highly opaque to short exposure wavelengths such as 193 nm, thereby resulting in poorly resolved images.
Compounding this problem is the inevitable fact that next generation lithography will resort to Extreme Ultraviolet (EUV) lithography. By utilizing extreme ultraviolet (EUV) radiation in the range of 4.5-15 nm, it is possible to produce features smaller than 0.18 μm. The resolution and therefore, the minimum feature size that can be obtained with EUV is a factor of 2-6 times better than with the present deep-UV or 193 nm lithography. However, as will be discussed below, other features of the projection lithography process have impeded the use of shorter wavelengths.
Unfortunately, most photoresist materials absorb extreme ultraviolet (EUV) radiation strongly in the range of 4.5-15 nm. While this is advantageous from the standpoint of resist speed (i.e. the exposure dose required to form a pattern) and the associated printing rate, it poses a serious problem for projection lithographic methods that employ EUV radiation because of highly nonuniform absorption of this radiation through the photoresist thickness. In present photoresist materials, EUV radiation will not penetrate much beyond a film thickness of 0.15 or 0.20 μm. Yet, to fabricate holes and other structures in semiconductor materials such as silicon, as well as metals, or dielectrics, the photoresist layer must be sufficiently thick, preferably in the range of 0.5-1.0 μm, to withstand etching and other processing steps.
Accordingly, in order to make use of the increased resolution afforded by the use of EUV radiation in the processing and fabrication of small structures, photoresists need to be developed that can be used in conjunction with high resolution EUV radiation and yet are compatible with conventional lithographic processing methods.
Therefore, with the impending widespread use of EUV lithography, new photoresists that satisfy the limitations inherent with EUV use are needed. Further, there is a need for new photoresists which can be used with EUV radiation and longer wavelength radiation sources as well.